1. Field of Invention
The invention relates to a control system for a hybrid vehicle which includes first and second drive units. The first drive unit has an engine, a first motor, and a planetary gear for power distribution. The second drive unit has a second motor and a stepped transmission which is interposed between the second motor and a transmission shaft and which transmits a driving force of the first and second drive units to drive wheels. More particularly, the invention relates to a control system for a hybrid vehicle, that is capable of outputting a driver request output to the drive wheels when the shifting of the stepped transmission is completed or while shifting is being executed.
2. Description of Related Art
Recently, various types of hybrid vehicles have been proposed which take into consideration effects on the environment and which aim to improve fuel economy. Among the hybrid vehicles, so called dual-motor power-split-type parallel hybrid vehicles have been proposed, for example, in Japanese Patent Application Laid-Open No. 08-207601. In this type of hybrid vehicle, one rotation element of a planetary gear is connected to an output shaft of an engine, the other two rotation elements are connected to a transmission shaft coupled to a first motor and drive wheels, and a second motor is connected to the transmission shaft.
In the hybrid vehicle as described above, while the first motor generates electric power by receiving a part of a driving force of the engine, it generates a reaction force in one of the rotation elements of the planetary gear. At the same time, a residual driving force is output to the transmission shaft via the rotation element of the planetary gear where the reaction force is generated. Further, the driving force from the second motor can be output to the transmission shaft. More specifically, control of the first motor enables (i) non-stepped control of a rotational speed of the engine and (ii) drive of the engine in an efficient region. At the same time, control of the second motor enables output to the drive wheels to be controlled. In other words, control of the first and second motors enables output to the engine to be driven efficiently and the driving force to be output to the drive wheels in accordance with an output requested by a driver (hereinafter referred to as “driver request output”).
Further, in the conventional hybrid vehicle as described above, the second motor is coupled to the drive wheels, that is, a rotational speed of the second motor increases as a vehicle speed increases. In general, when a motor rotates at a high speed, an output of the motor decreases as the rotational speed increases. For example, when the above vehicle is driving in a middle-to-high-speed range, the rotational speed of the second motor becomes high, and the output of the second motor decreases. Further, a vehicle requiring a large output (such as engine displacement of 3000 or 4000 cc) also requires a large capacity (or output) of the second motor. Therefore, in order to respond to a request for a larger output at a high rotational speed of the motor, the size of the second motor needs to be increased, which impairs mountability in the vehicle.